Premier Gladys Berejiklian has been accused of lying about her knowledge of the real reasons for a $549 million blowout to the cost of the Sydney light rail project, but refuses to release a document that would set the record straight.

Fairfax Media on Thursday revealed that in October 2014 Ms Berejiklian, as transport minister, was sent information contained in a briefing paper from Transport for NSW about the cost increase.

Premier defends her light rail record

NSW Premier Gladys Berejiklian has been forced to defend her management of a cost blowout of Sydney's light rail project in 2014 when she was Transport Minister.

Two months later, in December, Ms Berejiklian issued a statement on the blowout, attributing it to improvements in the project.

But an auditor-general's report in December last year said this was incorrect.

The auditor-general's report said the bulk of the increase – from $1.6 billion to $2.1 billion - was due to errors in the business case and that this had been reported to the government by October 2014.

In parliament, Ms Berejiklian refused to answer questions about whether she issued false information about the reason for the cost increase in her media release.

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She also refused to say whether she would release the briefing document, which has been deemed to contain cabinet information and therefore exempt from release under information access laws to Fairfax Media.

"I'm proud of the light rail project, I'm proud of all of our projects," she said.